Art of electrolytic cells comprising film-forming electrodes



' Jl lly 28, 1942. ROBINSON 2,290,996

ART OF ELECTROLYTIC CELLS COMPRISING FILM-FORMING ELECTRODES Filed July 16, 1950 3 nventor Pres-fan Fob/mar;

Gttorneg Patented July 28, 1942 UNITED STATES PATENT OFFICE ART L ELECTROLYTIC CELLS COMPRISING FILM-FORMING ELECTRODES Preston Robinson, North Adams, Mala, assignor to Sprague Specialties Mass., a corporation of Massachusetts Compa Q n Application July 16, 1930, Serial No. 488,488

19 Claims. (Cl. 175-315) become covered with a hard film. This film oilers a high resistance to the flow of current as long as the aluminum is the anode. However. it the aluminum is made the cathode, the film ofi'ers practically no resistance to the passage of the current.

Electrolytic cells of the above type are widely used as electrolytic condensers, rectifiers and lightning arresters.

Such electrolytic cells as stated comprise an aluminum anode, a suitable aqueous electrolyte,- for instance, a solution of borax and boric acid and the film is formed by subjecting the cell to gradually increasing electrical potentials, whereby the current fiow is properly regulated; the film which consists ofa partially hydrated aluminumoxide is thereby gradually formed on the surface of the aluminum, all this being well known to the art. When using such cells as static condensers, it is relied upon that the film once properly formed will remain unchanged and retain its high resistance to voltages of proper polarity as long as their values do not exceed the maximum forming voltage.

This is true to a great extent when the condenser is in continuous operation under the proper voltage conditions, as in such case the film does not dissolve in the electrolyte. However, when the condenser is idle, a slow dissolution of the film in the electrolyte takes place, which is highly objectionable.

For instance, when electrolytic condensers are used in filter circuits of radio receiving sets, the periods of idleness are long compared with the periods of operation.

When alter a'perlod of idleness the condenser is again placed in operation. a current surge takes place due to two causes. One cause is the loss of charge of the condenser due to leakage; this also takes place with other types of condensers, as for instance, paper impregnated condensers. However, the surge due to this cause being a transient current of very short duration, can be disregarded as a rule,

The second cause for a current surge is specific with electrolytic condensers and is due to the above referred to dissolution of the film. When such condensers, after a prolonged idleness are placed again in operation, the film, having been partly dissolved. oilers a reduced resistance to the current flow. While this current flow gradually rebuilds the dissolved film, such rebuilding requires considerable time and if not otherwise .checked, a relatively large current flow will persist for a considerable time.

The amplitude of the'current will be determined by the total resistance of the circuit, which in additionto the condenser, also comprises other devices. These devices may be damaged by the excessive surge of the condenser, unless their resistance is made high enough to prevent such. However, to provide these devices with sufiicient resistance is not always practicable, and even if feasible will require the use of devices of higher resistance than would be desirable for best operating characteristics of the circuit.

Not only is this current surge highly objectionable, but the repair of the film due to the sudden application of the full operating voltage will be only partial, as complete repair would require a gradual slow raising of the voltage, similar to the initial forming process. In fact, after longer periods of idleness, the initial quality of the film could be only restored by repeated formation.

Thus, besides causing an objectionable surge the condensers deteriorate during idleness due to the dissolution of the film in the electrolyte,

which is evidenced by a decrease in breakdown voltage and an increase in leakage current. For instance, present day condensers having a breakdown voltage of 450 volts after their formation. when'left idle for a few days, show a breakdown voltage of about 425 volts only. Thus roughly stated in a few days 25 volts of film is dissolved in the electrolyte. For longer periods of idleness, dissolution of the film and reduction in breakdown voltage is correspondingly greater.

Thus the slow dissolution of the film in periods of idleness oi the condenser has three drawbacks, namely, excessive currentv surge, lowering of breakdown voltage, and increasing of leakage current.

The object of my present invention is therefore to prevent dissolution of the film while the condenser is out of operation and thus to obtain condensers which afterlong periods of idleness-when again placed in operation-shall not cause excessive surge and shall retain their initial breakdown voltage and leakage resistance.

I have found that dissolution of the film during periods of idleness can be prevented to a 6021- siderable extent by adding to the aqueous electrolyte an excess of solid material of the same constituency as the film and thereby saturate the electrolyte with this material. In the case of aluminum electrodes this solid is aluminumoxide or aluminum-hydroxide..

However, in practice, the condenser is subjected to variations of the ambient temperature and to temperature rises during its operation. The solubility of the film in the electrolyte'is as a rule greater at the higher temperatures than at the lower temperatures, with the result that the electrolyte which is saturated at a lower temperature and therefore would not attack the film, at a higher temperature becomes unsaturated and will become saturated partly by dissolving the added solid, but partly also by dissolving the film.

By providing an excess of solid as stated and so forming the film, that the electrolyte is alternately heated and cooled during the forming process, I have been able partially to take care of this objection.

However, by a further improvement, explained hereafter, I have been able to prevent altogether dissolution of the film in the electrolyte, irrespective of the duration of idleness of the condenser, and irrespective of temperature changes during either operation or idleness.

Aluminum oxide and hydroxide exists in various forms of different solubility. For instance, in some forms, as ruby or corundum, aluminum hydroxide is very slow to dissolve in any solvent; on the other hand, in the form of hydro -gelatinous precipitates, it dissolves very ragidly in any good solvent. The partially hydrated aluminum oxide constituting the film, while more speedily soluble than ruby or corundum, has a much lower rate of solubility than the gelatinous aluminum hydroxide.

By adding such gelatinous substance to the electrolyte and saturating it at a low temperature, it will remain saturated when the temperature is raised, whereby the excess aluminum-oxide for saturation will be supplied by the gelatinous substance only and the film will not be attacked. In practice, I add the hydrous-gelatinous precipitate to the electrolyte, prior to the assembly of the condenser.

Or I may add to the electrolyte, a solution containing aluminum which when added will cause an excess of this gelatinous precipitate to be thrown down.

The advantage of my invention also manifests itself during the original forming process of the condensers, and condensers having a pre-saturated electrolyte and provided with an excess of gelatinous precipitates show a film, of greater uniformity than electrolytic condensers manufactured in the usual way. Condensers manufactured by my process have a leakage current of about 0.04 mil per microfarad as against 0.2 milllamp. for the present-day condensers.

At the same time, the initial breakdown voltwe of such condensers is almost identical with their forming voltage. This advantage is probably due to the fact that temperature changes which occur during formation do not tend to dissolve the film in the electrolyte.

The most important advantage of my process, however, is that with condensers manufactured in accordance therewith, after any period of idleness which may occur in practice, the current surge is almost negligible and the condensers retain their initial breakdown voltage and insulating resistance.

In the drawing forming part of this specification, the single figure is a partly sectionizcd side view of an electrolytic condenser embodying my invention.

The container i0 forms one of the electrodes of the condenser and may be of filming or nonfilming metal,

Provided in the container I0 is an electrode ii of film-forming metal, for instance, of aluminum, shown as a corrugated tube, although other forms of electrodes may be used. The film on the electrode is preferably formed in an electrolyte l6 which has added to it a substance having the same constituency as the film and being in the case of aluminum a partly hydrated aluminum oxide. The forming electrolyte is preferably saturated with such a substance and comprises also an excess of a solid thereof, and the electrolyte is alternately heated and cooled during the forming process. The electrolyte 16 consists of a solution of weak acid, and the salt of a weak acid, for instance, of an aqueous solution of borax and boric acid. The electrolyte also contains a substance to protect from attack by the electrolyte the film of the filming electrode, for instance, an hydrous gelatinous precipitate of aluminum oxide which is more soluble than the film and with which the electrolyte is saturated, preferably an excess of such precipitate being also present in the electrolyte.

The container I0 is closed by a cover H of insulating material through which projects a threaded extension I! of the electrode I3, said extension being provided with nuts l5-l5 to form one of the outside terminals of the condenser, the container IO forming the other terminal thereof.

The cover I! is provided with a vent l8 and a peripheral gasket l9, around which the free end of the container III is crimped. Preferably sealing means (not shown) are also provided between the protruding end of the electrode l3 and the cover l2.

While I have described my invention in its application to electrolytic condensers, I do not wish to be limited to such application, as my invention is equally well applicable to other electrolytic devices with film-forming electrodes; and while I have assumed aluminum as the material for the film-forming electrodes, other film-forming metals, for instance, tantalum, molybdenum, etc., may be substituted. Therefore I do not wish to be limited to the illustrated example of my application, but wish the appended claims to be construed as broad as permissible in view of the prior ar Claims to the introduction into the electrolyte of a substance other than of the same constituency as the film are being made in my copending application filed January 9, 1933.

What I therefore claim and desire to secure by Letters Patent is:

In the manufactur of electrolytic devices comprising a film-forming aluminum electrode adapted to form an aluminum oxide film and an electrolyte comprising a weak acid and a salt of a weak acid, the process which comprises the saturation of the electrolyte prior to use with a substance of the same constituency as the film.

2. In the manufacture of electrolytic devices comprising an aluminum electrode adapted to form an aluminum oxide film and a liquid electrolyte comprising a weak acid and a salt of a weak acid, the process which comprises the satv uration of the electrolyte with a substance of the same constituency as the film prior to the assembly oi the electrolytic device, said substance being more readily soluble in the electrolyte than the film.

3. In the manufacture of electrolytic devices comprising an aluminum electrode adapted to form an aluminum oxide film and a liquid elec- 1o rolyte comprising a weak acid and a salt of a weak acid, the process which comprises the saturation of the electrolyte with a substance of the same constituency as the film prior to the assembly of th electrolytic device, said substance being a hydrous gelatinous precipitate of the film substance.

4. In the manufacture of electrolytic condensers comprising an aluminum electrode and an aqueous electrolyte comprising a weak acid and a salt of a weak acid, the process which comprises the addition to the electrolyte of a gelatinous precipitate of aluminum-hydroxide, prior to the assembly of the electrolytic device.

5. An electrolytic device comprising prior to use a film-forming aluminum electrode adapted to form an aluminum oxide film and a liquid electrolyte comprising a mixture or a weak acid and a salt of a weak acid, said electrolyte being saturated with a substance of the same constituency as the film and also comprising an excess of said substance. I

6. An electrolytic condenser comprising a filmforming aluminu'm electrode adapted to form an aluminum oxide film and a liquid electrolyte comprising a mixture of a weak acid and a salt of a weak acid, said electrolyte comprising a substance of the same constituency as the film, said substance being more soluble in the electrolyte than the film. I

7. An electrolytic condenser comprising an aluminum electrode adapted to form an aluminum oxide film and a liquid electrolyte comprising a weak acid and a salt of a weak acid saturated with a substance of the same constituency as the film, said electrolyte also comprising a hydrous-gelatinous precipitate of the film substance.

8. An. electrolytic condenser having an aluminum anode and an aqueous electrolyte saturated with aluminum-hydroxide and comprising an excess of gelatinous precipitate of aluminumhydroxide.

9. In the manufacture of electrolytic devices comprising an aluminum electrode adapted to form an aluminum oxide film and an electrolyte comprising a weak acid and a salt of a weak acid, the process which comprises prior to the formation of the film saturation of the electrolyte with a substance of the same constituency as the film.

and alternately heating and cooling the electrolytic device during the formation of the film.

10. In the manufacture of electrolytic devices comprising an aluminum electrode adapted to form an aluminum oxide film and an electrolyte comprising a weak acid and a salt of a. weak acid, the process which comprises saturation of the electrolyte with a substance of the same constituency as the film and the addition of an excess thereof prior to the assembly of the device, and alternately heating and cooling the electrolytic device during the formation of the film.

11. An electrolytic device comprising prior to use a film-forming aluminum electrode adapted to form an aluminum oxide film and an electrolyte comprising a mixture of a weak acid and a salt of a weak acid, and saturated with a substance of the same constituency as the film.

12. An electrolyte for an electrolytic device with an aluminum electrode adapted to form an aluminum oxide film comprising prior to use borax and boric acid and a substance of the same constituency as th film.

13. An electrolyte for an electrolytic device with an aluminum electrode adapted to form an aluminum oxide .film, comprising prior to use a weak acid and a salt of a weak acid and a substance of the same constituency as the film.

14. The method of forming and operating electrolytic condensers having aluminum anodes adapted to form an aluminum oxide film to obtam a low power, comprising immersing the anodes in a film-forming electrolyte of a weak acid. and a salt of a weak acid containing a sub- "stantial amount of colloidal aluminum-hydroxide, forming films on the anodes so immersed, and thereafter using the anodes in a condenser having an electrolyte of a weak acid and a salt of a weak acid containing a substantial amount of colloidal aluminum-hydroxide.

15. An electrolyte for an electrolytic device with an aluminum electrode adapted to form an aluminum-oxide film, comprising prior to use, a weak acid and a salt of a weak acid and a substance embodying the same constituents as the film.

16. An electrolyte for an electrolytic device having an aluminum electrode provided with a film comprising in a major proportion a partially hydrated aluminum oxide, comprising prior to use, a weak acid and the salt of a, weak acid and a substance embodying the same constituents as the film.

17. An electrolyte for an electrolytic device 45 having an electrode provided with an oxygenated film of aluminum, comprising prior to use a weak acid and the salt of a weak acid and a substance embodying the same constituents as the film.

o 18. An electrolytic condenser having a low power factor and comprising the combination with a film-forming electrolyte containing borax and boric acid and a substantial amount of colloidal aluminum-hydroxide, of aluminum anodes 65 having films formed in an electrolyte containing borax and boric acid and a substantial amount of such hydroxide.

19. The method of forming and operating an electrolytic condenser having an aluminum anode, comprising forming an aluminum oxide 60 film on the anode by immersing the anode in a film-forming electrolyte containing a substantial amount of a substance of the same constituency as the film, and thereafter using the anode in a 65 condenser having an electrolyte of a weak acid and a salt of a weak acid and a substance embodying the same constituents as the film.

PRESTON ROBINSON. 

